JP4772486B2 - High strength steel pipe for low temperature - Google Patents

Description

  The present invention has high strength and excellent weld metal with an American Petroleum Institute (API) standard of X80 or more (yield strength 551 MPa or more, tensile strength 620 MPa or more) and less than X120 (yield strength less than 840 MPa, tensile strength less than 900 MP). Further, the present invention relates to a steel pipe having a weld heat affected zone (HAZ) toughness and welded layer by layer from the inner and outer surfaces with a welding heat input of less than 70 kJ / cm.

  Conventionally, as a means of transporting natural gas, a gas field relatively close to the market is generally transported by pipeline, and a gas field far from the market is generally transported by LNG. In any case, since large capital investment is required, improvement of natural gas transportation has been performed on the premise of so-called large gas fields. Recently, new technologies such as CNG (compressed natural gas), GTL (chemical petroleum product), and DME (dimethyl ether) have been developed, and the movement to develop and effectively use conventional small and medium gas fields has become active.

The idea of compressing and transporting natural gas has been around for a long time and has already been put into practical use as a fuel tank for automobiles. For large-scale sea transport, there was an idea of the transport method itself for about 40 years ago, but in recent years, technological innovation has progressed against the background of increasing demand for natural gas worldwide due to environmental problems. It is approaching the stage of practical use.
For example, it has been proposed that CNG is stored in a cylinder tank at a low temperature of −29 ° C. and a high pressure of 13 MPa, thereby greatly reducing the weight of the tank and enabling efficient transportation. Steel pipes used in this cylinder tank are required to have a large diameter (42 inches or more), thick wall (19 mm or more), high strength of API standard X80 or more, and excellent welds at a low temperature of -30 ° C. Toughness is required. In particular, since steel pipes are used as cylinder tanks, so-called pressure vessels, excellent brittle fracture occurrence characteristics evaluated by a CTOD (Crack Tip Opening Displacement) test are required.

  So far, line pipes up to X80 in the API standard have been put into practical use, and studies have been made on the basis of X80 class line pipe manufacturing methods (Non-Patent Document 1 and Non-Patent Document 2). In particular, there is a problem in terms of HAZ toughness, and an epoch-making high-strength steel pipe for low temperature that overcomes these problems is desired.

  The HAZ toughness of low alloy steel is (1) grain size, (2) high carbon island martensite (MA), dispersed state of hardened phase such as upper bainite (Bu), (3) grain boundary brittleness. (4) It is controlled by various metallurgical factors such as elemental microsegregation. Among them, the size of the HAZ crystal grains is known to have a large effect on the low-temperature toughness, and many techniques for refining the HAZ structure have been developed and put into practical use.

  For example, means for finely dispersing TiN to improve HAZ toughness during high heat input welding of 490 MPa class high strength steel is disclosed (Non-patent Document 3). However, since these precipitates are exposed to a high temperature of 1400 ° C. or higher in the vicinity of the melting line, most of them are coarsened or dissolved, and the HAZ structure is coarsened to deteriorate the HAZ toughness.

  In response to this problem, the HAZ structure near the melting line is refined by finely dispersing Ti oxide in steel and generating intragranular acicular ferrite (hereinafter referred to as IGF) in the HAZ during welding. It is disclosed in Patent Document 1, Patent Document 2, and the like that HAZ toughness is improved. However, since the structure is not sufficiently refined only by the generation of IGF from Ti oxide and the HAZ toughness deteriorates, improvement of the HAZ toughness of a high-strength steel pipe of X80 or higher is strongly desired.

  Therefore, as a means to further improve HAZ toughness, in order to suppress the austenite (γ) grain growth of HAZ near the melting line heated above 1400 ° C., a fine oxide composed of Mg and Al is incorporated into the steel. A new technology that combines a technology in which a large number of particles are dispersed and 0.01 to 0.5 μm of TiN is deposited on the core and a technology to refine the inside of γ grains in the HAZ near the melting line by IGF generation is patented. It is disclosed in Document 3. However, even in these composite technologies, generation of grain boundary ferrite (GBF) in the vicinity of the melting line cannot be completely suppressed, and HAZ toughness is deteriorated. Therefore, a new technology is required.

Patent Document 4 and Patent Document 5 describe that HAZ toughness is improved in a steel sheet obtained by welding a steel sheet having a tensile strength of less than 620 MPa containing boron (B) with a large heat input of 70 kJ / cm or more. For this purpose, a technique is disclosed in which the amount of B necessary for fixing solute N present in the HAZ near the melting line as BN is covered by diffusion of B from the weld metal. However, in the case of a steel plate welded with a large heat input of 70 kJ / cm or more, the cooling rate after welding becomes slow, so that solid solution N can be fixed as BN by diffusion of B from the weld metal. However, generally, in a welding joint having a welding heat input of 70 kJ / cm or less, the solid solution N cannot be fixed as BN because the cooling rate after welding is high.
“NKK Technical Report” 138 (1992), pp. 24-31 The 7th offshore Mechanics Arctic Engineering (1988), volume V, pp.179-185 "Iron and Steel" Vol. 65, No. 8, issued in June 1979, p. 1232 Japanese Patent Laid-Open No. 63-210235 Japanese Patent Laid-Open No. 1-15321 JP 2002-212670 A JP 2003-138339 A JP 2005-2476 A

  The present invention provides a high-strength steel pipe of X80 or more and less than X120 having good HAZ toughness (brittle fracture resistance occurrence).

The gist of the present invention is as follows.
( 1 ) In mass%,
C: 0.03-0.10%, Si: 0.6% or less,
Mn: 0.8 to 2.5%, P: 0.015% or less,
S: 0.001 to 0.005%, Nb: 0.005 to 0.05%,
Ti: 0.005-0.030%, Al: 0.001-0.010%,
Mg: 0.0001 to 0.0050%, N: 0.001 to 0.006%,
O: 0.001 to 0.005%
Containing the balance being iron and unavoidable impurities, containing TiN of 0.01~0.5μm enclosing the oxide of Mg and Al 10000 / mm ² or more, and oxides and sulfides A base material containing 10 particles / mm ² or more of 0.5 to 10 μm particles containing 0.3% by mass or more of Mn in a composite form;
C: 0.03-0.10%, Si: 0.6% or less,
Mn: 0.8 to 2.5%, P: 0.015% or less,
S: 0.005% or less, Nb: 0.005-0.05%,
Ti: 0.005 to 0.03%, B: 0.0015 to 0.0050%,
Al: 0.05% or less, N: 0.001 to 0.01%,
O: 0.015-0.045%
In the steel pipe having a weld metal part composed of iron and unavoidable impurities in the balance, the old austenite grain size of the weld heat-affected zone in the steel pipe welded layer by layer from the inner and outer surfaces in the longitudinal direction is 150 μm or less, and Boron is present in the prior austenite grain boundary of the weld heat affected zone within 150 μm from the wire, and the brittle fracture characteristics in the weld zone are CTOD values at −30 ° C. of 0.58 mm or more, Charpy absorbed energy at −40 ° C. A low-temperature high-strength steel pipe having a tensile strength of 620 MPa or more and less than 900 MPa, wherein the value is 114 J or more .

( 2 ) In mass%,
C: 0.03-0.10%, Si: 0.6% or less,
Mn: 0.8 to 2.5%, P: 0.015% or less,
S: 0.001 to 0.005%, Nb: 0.005 to 0.05%,
Ti: 0.005-0.030%, Al: 0.001-0.010%,
Mg: 0.0001 to 0.0050%, N: 0.001 to 0.006%,
O: 0.001 to 0.005%
Ni: 0.1 to 1.0%, Cu: 0.1 to 1.2%,
Cr: 0.1 to 1.0%, Mo: 0.1 to 1.0%,
V: 0.01 to 0.1%, Ca: 0.0005 to 0.0050%
1 or 2 or more of the above, the balance is made of iron and inevitable impurities, and 0.01 to 0.5 μm of TiN containing an oxide of Mg and Al is contained at least 10,000 pieces / mm ² , And the base material which 0.5-10 micrometer particle | grains containing 0.3 mass% or more Mn in the form which oxide and sulfide compounded contain 10 pieces / mm < ² > or more,
C: 0.03-0.10%, Si: 0.6% or less,
Mn: 0.8 to 2.5%, P: 0.015% or less,
S: 0.005% or less, Nb: 0.005-0.05%,
Ti: 0.005 to 0.03%, B: 0.0015 to 0.0050%,
Al: 0.05% or less, N: 0.001 to 0.01%,
O: 0.015-0.045%
Ni: 0.1 to 2.0%, Cu: 0.1 to 2.0%,
Cr: 0.1 to 1.5%, Mo: 0.1 to 1.5%,
V: 0.01 to 0.1%, Mg: 0.0001 to 0.0050%,
Ca: 0.0005 to 0.0050%
The former austenite grain size of the weld heat-affected zone in steel pipes welded one layer at a time in the longitudinal direction from the inner and outer surfaces in a steel pipe containing one or more of the above and the balance being a welded metal part consisting of iron and inevitable impurities Is not more than 150 μm, boron is present in the former austenite grain boundary of the weld heat affected zone within 150 μm from the melting line, and the brittle fracture characteristics at −30 ° C. is a CTOD value at −30 ° C. of 0.58 mm or more, − A low-temperature high-strength steel pipe having a tensile strength of 620 MPa or more and less than 900 MPa , wherein the Charpy absorbed energy value at 40 ° C. is 114 J or more .

  By adopting a high strength steel pipe (API standard X80 or more and less than X120) with excellent HAZ toughness according to the present invention to a CNG transport ship, the safety of sea transport of CNG is remarkably improved and resources can be used effectively. It was.

Below, the high temperature steel pipe for low temperature of this invention is demonstrated in detail.
The feature of the present invention is that the amount of Mg, N and O is strictly limited to low C—Nb—Ti system, and Mg
And a base metal part containing a fine carbonitride containing an oxide composed of Al and a composite composed of an oxide and a sulfide, and a low C—Mn—Ni—Cr—Mo—B based weld metal High-strength steel pipe having good HAZ toughness in a welded steel pipe composed of HAZ in which the former austenite grain size of HAZ is 150 μm or less and solid solution B exists in the former austenite grain boundary within 150 μm from the melting line It is in.

  The low temperature toughness of low alloy steel is governed by various metallurgical factors such as (1) crystal grain size, (2) dispersion state such as MA and upper bainite (Bu). Among them, the size of HAZ crystal grains and M-A are known to have a large effect on low-temperature toughness.

In HAZ of a high-strength steel pipe, HAZ toughness tends to deteriorate because a large amount of MA harmful to toughness is generated. In order to eliminate the adverse effect of MA, which is harmful to toughness, HAZ crystal grains must be refined thoroughly. Thus, HAZ crystal grains can be refined by a combined effect of a technique for generating IGF from within the γ grains together with a technique for suppressing the austenite (γ) grains in the HAZ to 150 μm or less.

  However, when γ grain coarsening suppression technology is introduced in the vicinity of the melting line, the hardenability in the vicinity of the melting line is lowered, and coarse ferrite side plates (FSP) and Bu are easily generated from the grain boundary, and brittle fracture occurs. A stable CTOD value cannot be obtained in the CTOD test for evaluating the generation characteristics. Therefore, by combining solid-state B in the γ grain boundary within 150 μm from the melting line and combining the technology to suppress the formation of coarse FSP and Bu structure from the γ grain boundary, the HAZ toughness and weld metal The inventors have found that the toughness can be remarkably improved and have reached the present invention.

  By adding fine MgN in the steel to produce fine carbonitrides such as TiN containing Mg and Al oxides, the coarsening of γ grains in HAZ is suppressed, and Mg, Mn, and S are added. By generating IGF from the oxide / precipitate containing, crystal grains can be made finer and HAZ toughness can be improved. Fine carbonitrides such as TiN containing oxides composed of Mg and Al, and oxides / precipitates containing Mg, Mn, and S are chemically stable and do not dissolve even at high temperatures. And the production effect of IGF is maintained.

  In HAZ heated to 1400 ° C or higher near the melting line, chemically stable fine oxides are used as pinning particles, and oxides and sulfides of 0.5 µm or more are used as IGF production nuclei. A method for thoroughly refining the HAZ structure was studied.

First, it has been found that a small amount of Mg (Al) oxide of 0.01 to 0.05 μm is produced in a large amount by containing a small amount of Mg and Al. It has been clarified that since 0.01 to 0.5 μm of TiN is complex-precipitated with this fine (Mg, Al) oxide as a nucleus, an excellent pinning effect of γ grains can be maintained even at a high temperature of 1400 ° C. or higher. At this time, when 0.01 to 0.5 μm of TiN contained in the steel is less than 10,000 pieces / mm ² , the effect of suppressing the coarsening of γ grains becomes insufficient, and good HAZ toughness cannot be obtained. .
Therefore, it is necessary to contain 10000 pieces / mm ² or more of 0.01 to 0.5 μm of TiN containing an oxide composed of Mg and Al. Furthermore, in order to produce this TiN, it is necessary to add 0.0001% or more of Mg. If the amount of Mg added is too large, Mg-based oxides increase and low temperature toughness deteriorates, so the upper limit was limited to 0.0050%.

  Further, in order to generate a fine (Mg, Al) oxide serving as a nucleus of TiN, it is necessary to contain a trace amount of Al. However, the addition of Al produces coarse alumina clusters, which adversely affects low temperature toughness. For this reason, the content of Al is limited to 0.001 to 0.010%. If the amount of Al is 0.001% or more, a fine (Mg, Al) oxide can be generated.

Next, as a necessary requirement for oxides and sulfides that form the core of IGF generation, IGF is also generated in HAZ by controlling the number, size, and composition of oxide / sulfide complexes, and the HAZ structure Has been found to be refined and to improve the HAZ toughness.

First, the number of oxide / sulfide complexes that form IGF production nuclei should be at least 10 / mm ² . If the number of IGF transformation nuclei is less than 10 / mm ² , the HAZ structure is not sufficiently refined and good HAZ toughness cannot be obtained.

  Moreover, in order to function as a transformation nucleus of IGF, a size of 0.5 μm or more is necessary. If it is less than 0.5 μm, it does not function sufficiently as an IGF transformation nucleus, and the HAZ microstructure refinement effect cannot be obtained. On the other hand, in the case of a composite of oxide and sulfide exceeding 10 μm, it becomes a point of occurrence of brittle fracture, so that good HAZ toughness cannot be obtained.

Furthermore, in order to function as a transformation nucleus of IGF, it is necessary to contain 0.3% by mass or more of Mn. In the present invention, in order to generate fine particles effective for pinning γ grains at a high temperature of 1400 ° C. or higher, Mg, Al, Ti, which has a stronger deoxidizing power than Mn, is contained. It is difficult to contain. Therefore, it is necessary to complex precipitate sulfide containing Mn on the oxide. If such measures are taken, the Mn content in the composite particles can be stably increased to 0.3% or more, and can effectively function as an IGF transformation nucleus.
As a result of searching for conditions for complex precipitation of Mn-containing sulfide on the oxide, the Mn content in the oxide is important. The oxide when the Mn-containing sulfide is combined contains 10% or more of Mg.
On the other hand, the Mg content in the oxide which is not compounded with sulfide and exists alone is less than 10%. That is, by containing 10% or more of Mg in an oxide of 0.5 to 10 μm, it is possible to stably composite precipitate the Mn-containing sulfide. As a result, 10 / mm ² or more of 0.5-10 μm IGF transformation nuclei containing 0.3% or more of Mn in the form of a composite of oxide and sulfide can be secured. The amount of Mn in the oxide is measured by EMPA (Electron Probe Micro Analyzer).
When the amount of Mn in the oxide / sulfide composite is less than 0.3%, a sufficient IGF generation function cannot be obtained, and the HAZ structure is not refined.

The old γ grain size of the HAZ of the steel pipe formed by welding this steel sheet and welding one layer at a time in the longitudinal direction of the steel pipe can be suppressed to 150 μm or less, and IGF is also generated. However, not CTOD value Oite hardenability is stable and is easily generated coarse ferrite side plate (FSP) and Bu grain boundary decreases the HAZ within 150μm can be obtained from the fusion line. Therefore, stable CTOD characteristics, so-called excellent HAZ toughness, can be obtained by segregating solid solution B at γ grain boundaries within 150 μm from the melt line and suppressing the formation of coarse FSP and Bu structures from the γ grain boundaries. .
B segregated at γ grain boundaries within 150 μm from the melt line is α-ray track etching (A
This can be confirmed by the (TE) method. Since the base material does not intentionally contain B, 1
It is estimated that B segregated at γ grain boundaries within 50 μm is due to diffusion from the weld metal.
As described above, H at a low temperature having a tensile strength of 620 MPa or more and less than 900 MPa.
The present inventors have found a welded steel pipe excellent in AZ toughness and have reached the present invention.

That is, the feature of the present invention is that, as a steel pipe base material, Mg, N and O are added to a low C—Nb—Ti system.
A mother having a tensile strength of 620 MPa or more and less than 900 MPa containing a fine carbonitride containing an oxide composed of Mg and Al, and a composite composed of an oxide and a sulfide. 90 parts at 620 MPa or more in the material part and low C—Mn—Ni—Cr—Mo—B system
A welded steel pipe composed of a weld metal part having a tensile strength of less than 0 MPa and HAZ in which the prior austenite grain size of HAZ is 150 μm or less and solid solution B exists in the prior austenite grain boundaries within 150 microns from the melting line It is in.

The reason for limiting the components of the steel pipe base material will be described below.
C needs to be added in an amount of 0.03% or more in order to secure the strength of the base material and the HAZ. However, if it exceeds 0.10%, the toughness of the base metal and the HAZ is lowered and the weldability is deteriorated, so 0.10% was made the upper limit.

  Si is an element added for deoxidation and strength improvement, but if added in large amounts, the field weldability and the HAZ toughness deteriorate, so the upper limit was made 0.6%. For the deoxidation of steel, Ti alone is sufficient, and Si does not necessarily have to be added.

  Mn is an indispensable element for securing strength and low temperature toughness, and its lower limit is 0.8%. However, if Mn is too much, not only the hardenability of the steel is increased and the on-site weldability and HAZ toughness are deteriorated, but also the center segregation of continuously cast steel pieces is promoted and the low temperature toughness is also deteriorated. 5%.

  In the present invention, the amount of P which is an inevitable impurity is set to 0.015% or less. The main reason is to further improve the low temperature toughness of the base material and the HAZ. The reduction of the P content reduces the center segregation of the continuously cast slab, prevents the grain boundary fracture and improves the low temperature toughness.

  S is an important element in the present invention. In order to cause composite precipitation of sulfide on the oxide as the IGF transformation nucleus, it is desirable to contain 0.001% or more. However, if S exceeds 0.005%, the toughness of the base material and HAZ deteriorates, so 0.005% is made the upper limit.

  Nb not only suppresses recrystallization of ν during controlled rolling and refines the crystal grains, but also contributes to precipitation hardening and hardenability, and has the effect of strengthening the steel. To obtain this effect, a minimum of 0.005% Nb is required. However, if the amount of Nb is too large, the HAZ toughness deteriorates, so the upper limit was limited to 0.05%.

Ti forms fine TiN, suppresses coarsening of γ grains of HAZ during reheating of the slab,
It is an essential element in the present invention by refining the microstructure to improve the low temperature toughness of the base material and HAZ. In order to exert this effect, 0.005% or more must be added. Also,
If too much, TiN coarsening and precipitation hardening due to TiC occurs, and low temperature toughness deteriorates.
The upper limit was limited to 0.03%.

N forms TiN, suppresses the coarsening of γ grains of HAZ during reheating of the slab and the base material, HA
Improve the low temperature toughness of Z. The minimum amount required for this is 0.001%. But N
If the amount is too large, the HAZ toughness is deteriorated due to slab surface defects or solute N, so the upper limit needs to be limited to 0.006%.

O forms an ultrafine (Mg, Al) oxide and exhibits the effect of suppressing the coarsening of γ grains of HAZ. At the same time, it forms an Mg-containing oxide of 0.5 μm to 10 μm to form an IGF transformation in HAZ. Functions as a nucleus. In order to exhibit these functions, 0.001% or more of O is necessary. When O is less than 0.001%, it is difficult to secure ultrafine oxide of 10,000 / mm ² or more and 0.5-10 μm oxide of 10 / mm ² or more. However, if O exceeds 0.005%, a coarse oxide exceeding 10 μm is generated, which becomes a point of occurrence of brittle fracture in the base material and HAZ, so 0.005% was made the upper limit.

Next, the reason for adding Ni, Cu, Cr, Mo, V, and Ca will be described.
The main purpose of adding these elements to the basic component is to improve the properties such as strength and low temperature toughness without impairing the characteristics of the steel of the present invention. Therefore, the amount added is of a nature that should be restricted by itself.

  Ni improves the strength and low temperature toughness of the base metal without adversely affecting weldability and HAZ toughness, but the effect is low at less than 0.1%, and addition of more than 1.0% is not preferable for weldability. The upper limit was made 1.0%.

  Cu has substantially the same effect as Ni, and is also effective in corrosion resistance, resistance to hydrogen-induced cracking, and the like, and it is necessary to add 0.1% or more. However, if added excessively, Cu-cracks are generated during precipitation hardening due to precipitation hardening and HAZ toughness, and the upper limit was made 1.2%.

  Cr has the effect of increasing the strength of the base metal and the welded portion, and it is necessary to add 0.1% or more. However, if too much, field weldability and HAZ toughness are significantly deteriorated. For this reason, the upper limit of the Cr amount is set to 1.0%.

  Mo is an element that increases the strength of the base metal and the welded portion. However, if it exceeds 1.0%, the base metal, the HAZ toughness and the weldability are deteriorated similarly to Cr. Moreover, the effect is thin when added less than 0.1%.

  V has substantially the same effect as Nb, but the effect is much weaker than Nb. In order to exhibit the effect, addition of 0.01% or more is necessary. Further, the upper limit is allowable up to 0.1% from the viewpoint of on-site weldability and HAZ toughness.

Ca controls the form of sulfide (MnS) and improves low-temperature toughness (such as an increase in absorbed energy in the Charpy test), and also exhibits a remarkable effect in improving sour resistance. 0
. If it is less than 0005%, the effect is weak, and if added over 0.005%, CaO—C
aS is generated in large quantities to become clusters and large inclusions, not only harming the cleanliness of steel,
It also adversely affects on-site weldability. For this reason, the amount of Ca added is limited to 0.0005 to 0.0050%.

Next, the reason for limiting the components of the weld metal will be described.
In order to prevent hot cracking of the weld metal, the C content needs to be 0.03% or more. If it is less than 0.03%, δ solidification occurs in the process of solidification after welding, and hot cracking occurs. However, if the C content exceeds 0.10%, the low temperature toughness of the weld metal deteriorates, so the upper limit was made 0.10%.

  Si is an element added for deoxidation and strength improvement, but if added in a large amount, the low temperature toughness and on-site weldability deteriorate, so the upper limit was made 0.6%.

  Mn is an indispensable element for securing strength and low temperature toughness, and its lower limit is 0.8%. However, too much Mn increases the hardenability of the steel and degrades low temperature toughness and on-site weldability, so the upper limit was made 2.5%.

  In the present invention, the amount of P which is an inevitable impurity is set to 0.015% or less. The main reason for this is to further improve the low temperature toughness of the weld metal. Reduction of the P content prevents grain boundary fracture and improves low temperature toughness.

  The amount of S which is an inevitable impurity in the weld metal is set to 0.005% or less. The main reason for this is to further improve the low temperature toughness of the weld metal. Reduction of the amount of S has the effect of reducing MnS and improving ductility.

  Nb has the effect | action which strengthens steel and needs 0.005% or more. However, if Nb exceeds 0.05%, on-site weldability and low temperature toughness are adversely affected, so the upper limit was made 0.05%.

  Ti addition forms fine TiN and improves low temperature toughness. In order to exhibit such an effect of TiN, it is necessary to add at least 0.005% Ti. However, if the amount of Ti is too large, TiN coarsening and precipitation hardening due to TiC occur and the low temperature toughness deteriorates, so the upper limit must be limited to 0.03%.

B is an element that greatly increases the hardenability of steel in a very small amount. In the weld metal, since it exists as a solid solution B at the prior γ grain boundary in the weld metal, the hardenability of the grain boundary is increased, and a fine structure is formed and good low temperature toughness is obtained. Furthermore , it diffuses into the HAZ during cooling after welding and segregates as solute B at the γ grain boundaries within 150 μm from the melt line, contributing to refinement of the HAZ structure. In order to obtain such an effect, B needs to be at least 0.0015 %. On the other hand, if added excessively, not only the low temperature toughness is deteriorated, but also the hardenability improving effect of B may be lost, so the upper limit was made 0.0050%.

  Al usually has an effect as a deoxidizing element. However, if the Al content exceeds 0.05%, Al-based non-metallic inclusions increase to impair the cleanliness of the steel, so the upper limit was made 0.05%.

  N forms TiN and improves low temperature toughness. The minimum amount required for this is 0.001%. However, if the amount is too large, the low temperature toughness is deteriorated, so the upper limit must be suppressed to 0.01%.

  O forms an oxide in the weld metal, acts as a nucleus of intragranular transformed ferrite, and is effective in refining the structure. However, if the amount is too large, the low temperature toughness of the weld metal deteriorates and welding defects such as slag entrainment occur. For this reason, the lower limit of the amount of O is set to 0.015%, and the upper limit is set to 0.045%.

Next, the reason for adding Ni, Cu, Cr, Mo, V, and Ca will be described.
In addition to the basic components, the main purpose of adding these elements as necessary is to improve the properties such as strength and low-temperature toughness of the weld metal without impairing the excellent characteristics of the steel of the present invention. . Therefore, the amount of addition is a property that should be restricted by itself.

  The purpose of adding Ni is to increase the strength without deteriorating the low temperature toughness and on-site weldability. However, if the addition amount is too large, not only the economy but also the low temperature toughness is deteriorated, so the upper limit was made 2.0% and the lower limit was made 0.1%.

  Cu, like Ni, increases strength without deteriorating low-temperature toughness and on-site weldability. However, if added in excess, the low temperature toughness deteriorates, so the upper limit was made 2.0%. The lower limit of 0.1% of Cu is the minimum value at which the effect on the material due to addition becomes remarkable.

  Cr increases the strength, but if it is too much, the low temperature toughness and on-site weldability deteriorate significantly. For this reason, the upper limit of the Cr content is set to 1.5% and the lower limit is set to 0.1%.

  The reason for adding Mo is to improve the hardenability of the steel. In order to obtain this effect, Mo needs to be at least 0.1%, preferably 0.5%. However, excessive addition of Mo deteriorates low temperature toughness and on-site weldability, so the upper limit was made 1.5%.

  V has almost the same effect as Nb, but the effect is weaker than that of Nb. V causes strain-induced precipitation and increases the strength. The lower limit is 0.01%, and the upper limit is acceptable up to 0.1% from the viewpoint of on-site weldability and low temperature toughness.

  Mg controls the form of sulfide (MnS) and improves low-temperature toughness (such as an increase in absorbed energy in the Charpy test). However, if the amount of Mg is less than 0.0001%, there is no practical effect, and if it exceeds 0.0050%, welding defects are generated. Therefore, the Mg addition amount is limited to 0.0001 to 0.0050%.

  Ca controls the form of sulfide (MnS) and improves low-temperature toughness (such as an increase in absorbed energy in the Charpy test). However, if the amount of Ca is less than 0.0005%, there is no practical effect, and if added over 0.0050%, a large amount of CaO—CaS is generated, and a weld defect is generated. For this reason, Ca addition amount was limited to 0.0005 to 0.0050%.

  The welded steel pipe of the present invention can be manufactured by forming a steel plate and then welding one layer at a time in the longitudinal direction from the inner and outer surfaces.

Examples of the present invention will be described below.
Steel pipes were produced using steel sheets produced from billets of various steel components by the converter-continuous casting method, and various properties were investigated. The low temperature toughness of the steel pipe weld was evaluated using a Charpy test piece and a CTOD test piece after one-layer submerged arc welding of the inner and outer surfaces. The notch positions were the weld metal center and HAZ (50% each of weld metal and HAZ were included).

The γ particle size of the welded portion of the steel pipe was determined by measuring the 10 old γ particle sizes in contact with the melt line at the 1/4 position of the thickness of the steel tube and calculating the average value. As for the segregation of B in HAZ , the segregation of B to the old γ grain boundary was investigated by an α-ray track etching (ATE) method. The amount and size of TiN encapsulating an oxide composed of Mg and Al were 10,000 times, 10 electron micrographs were taken, and the average value was obtained. The composition, size, and quantity of the oxide / sulfide composite were determined by observing a 4 mm ² field of view with EPMA and a 1000 × optical microscope.

  The test conditions and results are shown in Table 1 (Table 1-1, Table 1-2), Table 2, and Table 3. Table 1 shows the chemical composition of the steel pipe base metal and the weld metal, Table 2 shows the number of oxides, the γ grain size of the welded part of the steel pipe, the presence or absence of B segregation to the γ grain boundary, and Table 3 the steel pipe. The mechanical properties of the base metal and the mechanical properties of the welded steel pipe are shown. As is apparent from the table, the steel pipe of the present invention has high strength (YS, TS), low temperature toughness, and weld zone toughness. On the other hand, the comparative steel has inadequate chemical components and conditions to be provided, and any of the characteristics is inferior.

Since the steel 11 has a small amount of S, the HAZ toughness is inferior. Steel 12 is inferior in HAZ toughness because the amount of Al in the base material is small. Since the steel 13 has a large amount of Al in the base material, the HAZ toughness is inferior. Steel 14 is inferior in HAZ toughness because the amount of Mg in the base material is small. Since the steel 15 has a large amount of Mg in the base material, the toughness of the base material is inferior. Since the steel 16 has a small amount of C in the weld metal, hot cracking of the weld metal occurs.
Since the steel 17 has too much C amount of the weld metal, the low temperature toughness of the weld metal is inferior. Steel 18 is inferior in HAZ toughness because it contains 0.01 to 0.5 μm of TiN containing an oxide composed of Mg and Al, that is, the number of pinning particles is small. Steel 19 is in the form of a composite of oxide and sulfide, and has a particle size of 0.5 to 10 μm containing 0.3% or more of Mn, that is, the number of IGF transformation nuclei, and therefore HAZ toughness is inferior.
Steel 20 has 0.01 to 0.5 μm TiN containing an oxide composed of Mg and Al, that is, the number of pinning particles is small, and the γ particle size in the HAZ of the steel pipe welded part exceeds 150 μm. Inferior. Steel 21 has a small amount of B in the weld metal, and HA has poor HAZ toughness because B does not segregate at the γ grain boundaries within 150 μm from the melt line of the steel pipe weld.

  Since the welded steel pipe of the present invention is excellent in low temperature toughness, it can be applied not only to a cylinder tank for CNG transportation but also to a pipeline in a cold region.

Claims (2)

% By mass
C: 0.03-0.10%, Si: 0.6% or less,
Mn: 0.8 to 2.5%, P: 0.015% or less,
S: 0.001 to 0.005%, Nb: 0.005 to 0.05%,
Ti: 0.005-0.030%, Al: 0.001-0.010%,
Mg: 0.0001 to 0.0050%, N: 0.001 to 0.006%,
O: 0.001 to 0.005%
Containing the balance being iron and unavoidable impurities, containing TiN of 0.01~0.5μm enclosing the oxide of Mg and Al 10000 / mm ² or more, and oxides and sulfides A base material containing 10 particles / mm ² or more of 0.5 to 10 μm particles containing 0.3% by mass or more of Mn in a composite form;
C: 0.03-0.10%, Si: 0.6% or less,
Mn: 0.8 to 2.5%, P: 0.015% or less,
S: 0.005% or less, Nb: 0.005-0.05%,
Ti: 0.005 to 0.03%, B: 0.0015 to 0.0050%,
Al: 0.05% or less, N: 0.001 to 0.01%,
O: 0.015-0.045%
In the steel pipe having a weld metal part composed of iron and unavoidable impurities in the balance, the old austenite grain size of the weld heat-affected zone in the steel pipe welded layer by layer from the inner and outer surfaces in the longitudinal direction is 150 μm or less, and Boron is present in the prior austenite grain boundary of the weld heat affected zone within 150 μm from the wire, and the brittle fracture characteristics in the weld zone are CTOD values at −30 ° C. of 0.58 mm or more, Charpy absorbed energy at −40 ° C. A low-temperature high-strength steel pipe having a tensile strength of 620 MPa or more and less than 900 MPa, wherein the value is 114 J or more . % By mass
C: 0.03-0.10%, Si: 0.6% or less,
Mn: 0.8 to 2.5%, P: 0.015% or less,
S: 0.001 to 0.005%, Nb: 0.005 to 0.05%,
Ti: 0.005-0.030%, Al: 0.001-0.010%,
Mg: 0.0001 to 0.0050%, N: 0.001 to 0.006%,
O: 0.001 to 0.005%
Ni: 0.1 to 1.0%, Cu: 0.1 to 1.2%,
Cr: 0.1 to 1.0%, Mo: 0.1 to 1.0%,
V: 0.01 to 0.1%, Ca: 0.0005 to 0.0050%
1 or 2 or more of the above, the balance is made of iron and inevitable impurities, and 0.01 to 0.5 μm of TiN containing an oxide of Mg and Al is contained at least 10,000 pieces / mm ² , And the base material which 0.5-10 micrometer particle | grains containing 0.3 mass% or more Mn in the form which oxide and sulfide compounded contain 10 pieces / mm < ² > or more,
C: 0.03-0.10%, Si: 0.6% or less,
Mn: 0.8 to 2.5%, P: 0.015% or less,
S: 0.005% or less, Nb: 0.005-0.05%,
Ti: 0.005 to 0.03%, B: 0.0015 to 0.0050%,
Al: 0.05% or less, N: 0.001 to 0.01%,
O: 0.015-0.045%
Ni: 0.1 to 2.0%, Cu: 0.1 to 2.0%,
Cr: 0.1 to 1.5%, Mo: 0.1 to 1.5%,
V: 0.01 to 0.1%, Mg: 0.0001 to 0.0050%,
Ca: 0.0005 to 0.0050%
The former austenite grain size of the weld heat-affected zone in steel pipes welded one layer at a time in the longitudinal direction from the inner and outer surfaces in a steel pipe containing one or more of the above and the balance being a welded metal part consisting of iron and inevitable impurities Is not more than 150 μm, boron is present in the former austenite grain boundary of the weld heat affected zone within 150 μm from the melting line, and the brittle fracture characteristics at −30 ° C. is a CTOD value at −30 ° C. of 0.58 mm or more, − A steel pipe for low temperature and high strength having a tensile strength of 620 MPa or more and less than 900 MPa , wherein the Charpy absorbed energy value at 40 ° C. is 114 J or more .